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Radioactive Dating If we somehow know what the original abundances were at time t 0,then by measuring the abundances now, we can calculate the time since t 0. Problems: We often do not know original abundances How do we define t 0 anyway??

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Radioactive Dating Definition of time t 0 : The moment at which a rock has solidified (crystallized). Key ingredient: We assume we have chemically heterogeneous patches of rock (if not, then we would only measure the time since element formation in previous generation of stars, which is nice, but not what we want; or if the system is not closed, we would not know initial abundances, so our measurement is useless). Before t 0 : We assume that the material has been well-mixed between the above mentioned patches, so that all isotopes of the same element have the same abundance ratios between the patches. Only the chemistry can create heterogeneous patches: but this only creates variations in abundances of different elements, not of their isotopes.

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Radioactive Dating After t 0 : We assume that chemistry has stopped. No exchange of elements nor of isotopes between patches. From now on the radioactive decay can change isotope ratios. For different patches this occurs with different rate, because of different chemical compositions. Example: U-Pb system: 238 U 206 Pb Halflife = 4.47 Gyr 235 U 207 Pb Halflife = 0.70 Gyr If the U/Pb ratio in one patch is high, then 206 Pb grows fast; if the U/Pb ratio is low, then 206 Pb grows slow. Same for 207 Pb but with 6x lower rates. By comparing two or more patches you get age since solidifying.

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Clair Cameron Patterson Clair Patterson was the first (1953) to use Pb-Pb dating to date the age of the Earth and the solar system bodies: Age = 4.5 Gyr (Now: 4.567±0.001 Gyr) He also discovered in the process that humanity was in the process of poisoning itself with leaded car fuel! His continued effort agains strong lobbies by the industry eventually led to the Environmental Protection Agency 1973, which stopped and eventually reversed the lead output into the food chain.

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Late Heavy Bomardment? Samples from the Apollo moon landings all seem to date from between 3.92 and 3.85 Gyr ago (remember: age of the solar system = 4.5 Gyr). Conclusion: Something dramatic must have happened: A lunar cataclism. But: could the Apollo samples all be polluted by a single large impact?

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Meteorites: Chondrules, matrix & CAIs

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Chondrules: A big mystery Chondrules are round once-molten droplets that have been flash-heated and very rapidly cooled (the whole process no longer than a few hours). Evidence for this quick process: –They still contain many volatile elements, which they would not, if they were heated over a long time. –Their textures require quick cooling Ideas: –Colliding planetesimals? –Shock heating in the nebula? –Lightning in the nebula? –Reconnection events in the nebula?